Astrophysics from Space Lecture 4: The extragalactic distance scale Prof. Dr. M. Baes (UGent) Prof. Dr. C. Waelkens (KUL) Academic year

The expanding Universe One of the most important cosmological discoveries: the Universe is expanding. Vesto SlipherGeorges LemaîtreEdwin Hubble

The expanding Universe One of the most important cosmological discoveries: the Universe is expanding.

Hubble’s law Hubble’s first determination resulted in H 0 = 500 km/s/Mpc Resulting age of the Universe: about 2 Gyr Radioactive dating of Earth rocks (> 3Gyr)

Hubble’s constant 1960s – 1970s: two groups advocating two distinct values (50 and 100 km/s/Mpc)

Cosmic distance ladder Each rung of the ladder provides information that can be used to determine distances at the next higher rung.

Fundamental distance measurements Methods in which distances are measured directly, with no physical assumptions about the nature of the object. Fundamental distance: scale of the Solar System Astronomical Unit (AU) Kepler’s third law: if we know the distance to one planet, all distances in the Solar System are known

Planetary parallax measurements Late 1800s: efforts concentrated on Venus at inferior conjunction (in particular during Venus transit) Early 1900s: Mars and Eros

Radar echo measurements From 1960s: radar echo measurements on Venus and other nearby planets and asteroids AU = ± 30 m

Stellar parallax measurements High astrometric precision is necessary to measure large distances Hipparcos: precision of milli-arcsec accurate distances out to several 100 pc also accurate distances to next distance ladder objects (Cepheids) Contribution of Hipparcos to the cosmic distance ladder has been crucial

Standard candles: cepheids

1908: Henrietta Leavitt discovers period-luminosity relation in cepheids (studying the LMC and SMC) 1915: Harlow Shapley uses cepheids to determine the size of the Milky Way 1924: Edwin Hubble uses cepheids to determine distance to Andromeda

Standard candles: type Ia SN White dwarfs in a binary system, where infalling matter pushes it over the Chandrasekhar limit. Consequence: thermonuclear explosion.

Standard candles: type Ia SN The width of the light curve correlates with the peak luminosity. Huge advantage: type Ia supernovae are extremely bright. Type Ia supernovae are ideal standard candles for cosmological studies

Standard candles: caution… (1)Calibration issues: what is the absolute magnitude (requires thorough definition of a class and enough members in that class) (2)Confusion with similar objects (different SN types, novae versus supernovae…) (3)Interstellar extinction (4)How standard are standard candles ? For example: there are two classes of cepheids…. Walter Baade

Secondary distance indicators Indirect distance indicators, often based on statistical relations in galaxies. To be used when no primary indicators can be used. Prime example: Tully-Fisher relation

HST Key Project HST Key Projects large observations projects with significant impact separate time budget (no competition) guaranteed completion in the early years HST Key Project on the Extragalactic Distance Scale measure direct distances to galaxies using cepheid variables use these distances to calibrate secondary distance indicators (TF) to probe structure of the Universe

HST Key Project Spectacular results obtained as soon as the optics were refurbished (late 1993). 8 observations of each target galaxy to detect cepheids with periods of 10 – 50 days. E.g. more than 80 cepheids discovered in M100.

HST Key Project Final result based on cepheids and cepheid-calibrated secondary methods: H 0 ≈ 72 ± 8 km/s/Mpc